#include <stdio.h>

int main(){

    double a = 2, b = 1;
    int n;
    double sum = 0;
    scanf("%d", &n);
    double temp;
    for (int i = 0; i < n; i++){
        sum += a/b;
        temp = a;
        a += b;
        b = temp;
    }
    printf("s=%.2lf", sum);
}